Presenilin antibodies detect neurofibrillary tangles in neurons of familial Alzheimer’s disease (AD) brains. A significant number of neocortical neurons from lamina III of familial Alzheimer’s disease (FAD) brains carrying the mutation M139I in presenilin 1 (PS1; A, B, C are from one case, D from a second) showed intracellular neurofibrillary tangle-like labeling with PS1 antibodies (labeling with (P1) is shown; a similar pattern was seen with NT1). Similar labeling was not detected in the same populations from control (see Fig. 5), sporadic AD (see Fig. 6), or other FAD brains (not shown).